EP-A 358 979 discloses aqueous dispersions of hydroxy-functional copolymers, particularly those of the kind known as secondary dispersions, which have become established as particularly suitable polyol components for aqueous two-component (2K) polyurethane varnishes. These products have been found appropriate especially for the wood coating sector, since varnishes based on secondary copolymer dispersions are distinguished by particularly good grain attenuation of the wood. Aqueous (2K) polyurethane varnishes based on secondary copolymer dispersions, used as binders in application sectors which allow only room-temperature drying, such as for the varnishing of wood-block flooring, for example, on the other hand, require too long a time until the varnish reaches its full load-withstanding capacity. Critical parameters for the load-withstanding capacity are considered to include the drying time and the time until sufficient resistance to long-term loading is acquired, such loading being that produced, for example, by heavy pieces of furniture (known as blocking resistance).
EP-A 358 979, DE-A 42 262 70 and EP-A 1 024 184 describe aqueous (2K) polyurethane varnishes based on secondary vinyl polymer dispersions and polyisocyanate crosslinkers that already have a good level of properties. The vinyl polymers serve in this case as an emulsifier for the polyisocyanates and thereby prevent unwanted isocyanate-water reaction. A disadvantage of these systems, however, is their deficient drying rate, which is vital for applications in the field of the varnishing of wood-block flooring.
DE-A 44 39 669 describes aqueous two-component polyurethane coating materials based on water-dilutable polyacrylate resins and polyisocyanates, which are applied as clearcoat material over a basecoat film and are distinguished by rapid attainment of a dust-dry and tack-free state and also by rapid volume drying. The coating materials described therein are used in particular in the OEM finishing or refinishing of car bodies. A disadvantage of these systems is their inadequate blocking resistance, which rules out their use in the wood coating sector.
For the wood varnishing sector, therefore, there continues to be a need for suitable coating materials which can be processed to (2K) polyurethane varnishes with good film optical qualities, hardness and chemical and mechanical resistance and which at the same time possess a sufficient drying rate and blocking resistance.